Current therapy for Alzheimer's disease (AD) involves the use of acetylcholinesterase (AChE) inhibitors to attempt to overcome the loss of acetylcholine. We raise the question as to whether AChE inhibitors would be significantly more effective in treating AD if given much earlier - before serious symptoms appear. We address this question theoretically by considering a positive-feedback model for the development of AD, and using that model to calculate how the time for starting treatment affects the time of onset of AD. The mathematical model we use involves positive feedback between the decreased concentration of acetylcholine and the increased concentration of beta- amyloid, and the AChE inhibitor we consider is donepezil (Aricept).